This invention relates to composting systems for treatment of organic waste in a manner accepted to be environmentally sustainable.
Each day a large amount of solid municipal domestic waste is produced. Of all solid municipal domestic waste produced in Australia approximately 50-55% consists of food and garden waste. Other components of the waste include paper (20%), plastics (6%), glass (10%), metals (5-7%) and other inorganics (10-15%). The organic fraction has a strong detrimental impact on the environment and may be hazardous.
Detriment results from the large volume of organic waste which may occupy 50-70% of landfill space. The waste is of a putrescible nature, thus making it a potential source of pathogenic organisms. A large volume of greenhouse gases, e.g. carbon dioxide and methane are released during uncontrolled decomposition. Finally, and significantly, decomposing organic matter causes odour, attracts pests and is a major contributor to groundwater pollution through dissolution and its role as a carrier or inorganic pollutants such as heavy metals. In certain cases such pollution may make groundwater unsuitable for safe use.
Therefore, one of the main challenges in any integrated waste management strategy is the appropriate and effective treatment of organic waste. The current practice of landfilling organic waste is rapidly becoming an inappropriate waste treatment practice and will be unsustainable in the long term.
Landfilling consumes large areas of land, results in low land value and is strongly objected to by residents. Consequently, landfilling is fast disappearing in major cities as a sole waste disposal strategy. In cities with low housing densities, transport costs may become prohibitive as suitable landfill space becomes available only well away from waste generation centres.
Organic waste nevertheless has considerable potential as a resource when stabilised through composting. It is high in organic matter and contains nutrients such as nitrogen (2%), phosphorus (0.5-0.7%), potassium (0.7-1.7%) and trace elements.
Composting is the process whereby organic matter is decomposed by a range of microorganisms using oxygen. The process is appropriate for treatment of. a combination of fibrous waste (e.g. green organics) and putrescible waste such as food waste, sewage sludge and industrial and commercial organic residues. Composting has the advantage of reducing the waste volume by 30-40%. In addition, product compost has significant benefits as a soil conditioner.
A typical composting process may comprise four stages. Each stage is characterised by the activity of different generations of bacteria, fungi, protozoa and actinomycetes. During each stage the microbes use original organic compounds present in the waste as well as by-products of the metabolism of the previous generation as a nutrient and energy source. Thus the organic matter decomposes until a stable humus is formed.
The incubation or mesophilic phase lasts for approximately 24 hours during which the organic matter is rapidly invaded by mesophilic composting organisms including bacteria, actinomycetes and fungi. These organisms thrive at a temperature of 25xc2x0-45xc2x0 C. The mesophilic organisms grow in this phase on the more easily assimilated substances present in the organic waste, for example: sugars, soluble protein, starch and organic acids.
The high metabolic activity of the organisms and the exothermic decomposition processes that result, in combination with the insulating properties of the composting material, causes the temperature to rise. The temperature rise strongly favours thermophilic sporogenous bacteria. The activity of these bacteria takes the process into the thermophilic phase.
During the thermophilic phase, organic matter is decomposed rapidly. Temperatures may reach 70xc2x0 C. in the core of the composting material. This is undesirable, since at this temperature most process participating microbes, including some thermophilics, are killed. This may considerably reduce the decomposition rate of organic material. About 45xc2x0-50xc2x0 C. is optimum and above 55xc2x0 C. is typically required for pathogen destruction, thus 55xc2x0 C. is accepted as an optimum temperature compromising between these factors, at which the decomposition rate is highest. These temperatures assist in accelerating the process and sanitising the material from pathogens, weed seeds and plant disease causing agents. This temperature, and below, allows the development of eumycetes and actinomycetes which are the main decomposers of long chain polymers, cellulose and lignin. The oxygen demand is very high in this phase and aeration is required. This phase may last for 2-3 weeks depending on aeration and substrate.
The cooling phase commences when there is insufficient exothermic organic substrate left to maintain the high temperature. Accordingly, water evaporation and heat convection cause temperature to drop. If the temperature drops below 45xc2x0 C. mesophilic bacteria and other organisms may reinvade the fresh compost. This phase may last a few days.
A maturation or stabilisation phase is required to allow the toxicity of fresh compost to fall to enable effective utilisation by plants. The activity of fungi, protozoa and actinomycetes may be highest during this phase, while bacterial activity slowly falls. At this stage, large polymers such as lignin and cellulose are decomposed and a humidification process sets in. The activity of actinomycetes produces the compound xe2x80x9cgeosminexe2x80x9d which gives matured compost a fresh earthy smell. Three to four weeks may be sufficient to enable completion of this phase.
It is an objective of the present invention to provide a composting system that may enable the respective phases of the composting process to be completed in an efficient manner, under conditions that enable the most advantageous conditions for aerobic microbial decomposition of an organic substrate to occur.
With this objective in view, the present invention provides a composting system comprising an area for holding compostable material; a mass of compostable material located in the area; the area and a weatherproof cover or structure for the area defining an environment in which composting occurs, and an environmental condition which is controlled to optimise the composting process.
In particular, the weatherproof cover is preferably to be secured or appropriately sealed to prevent ingress of water or other environmental, especially climatic, impacts detrimental to efficient composting such as excessive drying out. Entry of pests and vermin is also prevented. A flexible or modular construction for the cover is preferred, particularly a cover which is readily conformable to the volume of material to be composted. The volume of the environment is dictated primarily by the volume of compostable material to be treated.
In this respect, the invention is predicated on the discovery that the composting process is dependent upon a number of variables, the control of which ensures a more efficient composting process. Further, the cover prevents escape of odour, water ingress and generation of leachate. Leachate generation is a particular problem in open air composting systems such as windrow composting and static pile forced aeration composting which are strongly dependent on weather conditions. Where rainfall is high, water may drain through compost windrows, leaching nutrients and soluble organic matter from the compost. The generated leachate may usually require treatment before discharge to surface or ground water and compost quality is reduced.
For a first example, the control of aeration may be important to conducting the composting process. Aeration provides the oxygen necessary to sustain the aerobic organisms that promote composting. In a static pile, oxygen levels can drop to below 1% by volume and carbon dioxide levels can reach 20% by volume. Such levels may be inhibitive to composting.
Therefore, aeration means to aerate the mass of compostable material are to be included within the composting system. The aeration means may provide a variable controllable proportion of recycled process air and fresh air assisting in maintenance of the compostable mass moisture at desired levels importantly preventing drying out of compost, and may provide oxygen levels within the mass of 10-18% by volume.
Control over the level of carbon dioxide in the mass of compostable material may also be desirable. In this respect, air flow may be controlled such that carbon dioxide levels are maintained below 10% by volume.
In this respect, the O2 and CO2 levels are interlinked and add up to xc2x121%. Thus if oxygen is 15%, carbon dioxide is 6%.
Accordingly, the composting system may advantageously include means for maintaining an appropriate moisture level. In this respect, recirculation of spent process air or oxygen through the mass of compostable material may be conducted to cause a flow of moist air which maintains moisture in the mass at desired levels and a carbon dioxide level of approximately 15% by volume. Recirculation of air or aeration by other means may also assist in achievement of a homogeneous moisture level throughout the mass avoiding stratification or formation of dry spots which adversely affect composting. Fresh air may then be introduced by blower or other air compressing means to maintain a ratio of recycled air to fresh air, sufficient to maintain the desired oxygen carbon dioxide level. Alternatively, fresh air or oxygen may be introduced at any time to maintain a desired recycle air to fresh air ratio and/or carbon dioxide concentration.
This ratio may be maintained at the desired level in dependence upon monitored oxygen or carbon dioxide level but may also be controlled as a function of other composting process variables, such as the temperature of the core of the mass of compostable material. Suitable sensors may be provided for these purposes and the composting system may be under the control of a microprocessor or like device.
Where warm, moist air contained within the cover is mixed with fresh air and recirculated through a blower or like means, the temperature and moisture level of the air entering the core of the compost windrow increases preventing drying out and/or premature cooling.
The weatherproof cover may be sealed at its edge by a low cost sealing means such as sandbags, soil, a water jacket, beams rods or other means. It is important that the sealing be achieved in a manner that enables effective control over the microenvironment within the weatherproof cover. Space may be at a premium. In this respect, the system is ideally designed to exclude climatic influences over the composting process, chiefly drying influences and excess rainfall which may base excess moisture and leachate generation or prolonged hot and dry conditions which may dry the compost to a point where microbiological activity ceases. The system is also advantageously flexible to suit variation in the mass of material to be treated and sealing means may be selected with this in view.
A clear manifestation of climatic influence is rainfall. Rainfall may markedly interfere with a composting process because the level of moisture in the mass of compostable material is an important process parameter. While the composting microbes may require a minimum moisture content of about 40% to avoid reduced activity, levels of moisture above about 60% may lead to the occurrence of anaerobic conditions which change the process from a composting process to a fermentation (or rotting) process. This will occur when the pores in the substrate fill with water to an extent that effective aeration is impaired. Further, excess water may be a cause of leachate generation, undesirable for environmental reasons, for example, base metal pollution and uncontrolled nutrient loss to the environment. Hence, the weatherproof cover is ideally to be a material that prevents ingress of water to the composting system due to rainfall and which prevents excess moisture loss due to drying, especially of the edges, during hot weather. Ideally, the material should facilitate collection of moist spent air and dry fresh air.
In addition, temperature may be important. Composting involves a thermophilic stage and drop in ambient temperature may effect this. Thus the material may be of a kind that prevents ingress of ambient air to the mass of compostable material and escape of odour. If necessary, provision for addition of make up fresh air may be made to minimise the effects of ambient temperature.
Typically, the system through controlled aeration and exclusion of water enables compost to be held at a sufficient temperature, defined by some standards as 55xc2x0 C. or higher, for a sufficient period, a few, typically three, days to kill pathogenic organisms present in the material or biosolids.
Pathogens, both plant and human, are inherent to most types of organic wastes. In order to minimise risks to public health and flora, i.e. crops, such materials must typically be processed such that substantially complete pathogen destruction is achieved.
The provision of air circulation through the compostable material may assist in avoiding a situation where low temperature zones are formed at the base of a pile or a periphery of a windrow where excessive heat loss to the atmosphere and lack of insulation may prohibit temperatures reaching thermophilic levels. Drying out may also occur at this periphery in open windows.
Product compost which may typically have less than ten (10) total coliforms per gram compost in comparison with ARMCANZ guidelines (see Agriculture and Resource Management Council of Australia and New Zealand Water Technology Committee, Guidelines for Sewage Systemsxe2x80x94Biosolids Management Occasional Paper WTC No 1/95 October, 1995) which specify that Class A compost may contain a maximum pathogen concentration of 100 thermotolerant coliforms per gram compost.
Typically, the compostable material may be turned once during the process (duration usually approximately 8 weeks) although turning may be conducted more frequently. However, frequent turning is undesirable as labour and equipment costs may be increased and it is not an aspect of a preferred embodiment of the present invention.
In a further aspect, the present invention provides a composting method comprising delivering a mass of compostable material to an area for holding compostable material.
The area having a weather proof cover or structure for the area defining an environment in which composting occurs, an environmental condition of which is controlled to promote the composting process.
The composting system and method of the present invention may provide a number of advantages. Firstly, the system allows control over odour, the xe2x80x9cballoonxe2x80x9d formed by the preferably flexible weather proof cover around the mass of the compostable material including biosolids preventing odour emission. Recycled air may be deodorised by the compost mass acting as a biofilter and excess air which requires venting may be led through a biofilter for substantially complete odour removal. Leachates are not generated in any significant amounts and may be contained and not released to the surrounding environment, at least in an untreated state. In addition, the xe2x80x9cballoonxe2x80x9d creates a homogeneous microenvironment which is controllable to the benefit of efficient composting. The control achieved over moisture level and pathogens may allow, for example, a more rapid composting rate. In addition, the system offers a benefit of low cost with the various components available in most locations at low cost. Low cost also offers the advantage of plant mobility with low capital risk when the system is moved from place to place.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.